ARM: ux500: Rid ignored return value of regulator_enable() compiler warning

arch/arm/mach-ux500/board-mop500.c: In function ‘mop500_prox_activate’:
arch/arm/mach-ux500/board-mop500.c:406:18: warning: ignoring return value of
        ‘regulator_enable’, declared with attribute warn_unused_result
        [-Wunused-result]

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Olof Johansson <olof@lixom.net>